Methods for echo ranging underwater have generally required placement underwater of sound generating and receiving devices, for example as disclosed in Canadian Patent No. 907,720 issued Aug 15, 1972. The same is true of seismic exploration methods by sound refraction in marine environments, for example, as disclosed in Canadian Patent No. 1,019,433 issued Oct. 18, 1978; U.S. Pat. No. 4,242,740 issued Dec. 30, 1980 and U.S. Pat. No. 4,139,074 issued Feb. 13, 1979.
Arctic exploration presents problems not encountered in general marine depth sounding or exploration. As the last above-mentioned United States patent explains:
Arctic exploration methods and equipment must overcome the problems of the ice-water terrain and frigid arctic temperatures. Since the ice most resembles a land terrain, heretofore methods of seismic exploration used on ice were similar to those of land exploration. This method involved using explosive charges in the form of point charges or a dispersed charge in the water beneath the ice depending on the distribution of energy desired. A recording truck towing a land cable having sections of geophones attached thereto is placed within an appropriate range to detect the seismic signal data created by the explosive charges. The two major problems with the land exploration method used on ice terrain are environmental and economic in nature. Environmentally, use of explosive charges in the water beneath the ice is found to be harmful to the water life therein. Economically, the use of explosive charges over other methods of generating seismic sources is far more expensive. PA0 In marine seismic exploration, seismic events are created by detonating an explosive charge or by generating gaseous explosions using compressed air guns. There are also electrical discharge systems using an underwater spark to create an acoustic pulse, but these are mostly broadband and so generally inefficient with reference to narrow frequency bands of interest in seismic exploration. The seismic signal data generated by the seismic disturbance is detected by hydrophones attached to a streamer towed by a boat through the water PA0 Considering the terrain involved in arctic exploration, it is impossible to use a pure marine system. And, for the reasons described above, a pure land exploration system is not practical for arctic exploration. Although an air gun has been used in arctic regions to discharge air creating a seismic disturbance in the water beneath the ice and the seismic signal data from such disturbances has been recorded by geophones, this technique has been generally unreliable because of difficulties encountered in the operation of the air gun under the extreme conditions encountered in arctic regions. Also, air gun handling in both marine and primitive arctic methods heretofore employed has been manually accomplished, this giving rise to a possible safety hazard and increasing the time cycle of exploration, which is defined by the time needed to create and record a seismic disturbance.
This is a good summary of the problems, at least those facing seismic exploration, in Arctic waters
A different but not unrelated subject is that of the sounding of Arctic waters to determine depth for purposes of charting the waters. The Canadian Hydrographic Service of the Government of Canada has conducted Arctic water soundings through the ice by means of acoustic methods based on a common principle. The ice surface, sometimes snow covered, has an electro-acoustic transducer applied thereto which transmits sound signals into the ice and water and which receives the echo from the sea floor. The delay from transmission to reception is a measure of water depth. The transducers have been deployed both manually from a tracked vehicle and mechanically pressed against the ice or the snow by actuators placed on tracked vehicles and helicopters.
As will be appreciated, even the method of sounding using mere surface contact with the ice is slow, for the vehicle, whether tracked or helicopter, must stop while a sounding is being conducted. Thus the method is slow.
In a paper published Mar. 1983 in the Journal of the Acoustical Society of America 73(3) by G. Hickman and J. Edwards, titled "Laser-acoustic measurements for remotely determining bathymetry in shallow turbid waters", the authors discuss experiments conducted using an infrared C0.sub.2 laser transmitter and a highly sensitive microphone receiver located in the air. The pulsed laser beam generated acoustic signals in water having pulse widths of 20 to 30 microseconds, the echo of which was detected by the microphone. The paper also explains, in addition to pulsed laser sound generation, detection techniques, signal path geometries and related matters. The paper is incorporated herein in its entirety by reference.
It is not known whether ice-covered Arctic waters can be sounded by the laser technique. Furthermore, it is reasonable to expect that snow-covered Arctic waters could not be sounded by that technique. Accordingly, it is desirable to develop a reliable technique for the depth sounding of waters, whether ice-free, ice-covered or snow-covered. covered.
Depth charting of waters requires a non-contact method in order to speed up the process. Yet the laser excitation method is either not practical or reliable enough, or not usable altogether for ice-covered waters, and certainly not for snow-covered (over-ice) waters, because the snow reflects the energy of the laser impulse. This is unfortunate because the sound impulse produced by pulsed laser is narrow and therefore well defined.